Signal processing in audio equipment often includes converting all of a plurality of different sound sources to analog signals, selecting one analog signal from the plurality of analog signals by a selecting circuit, converting the analog signal to a digital signal by an analog-to-digital (A/D) conversion and then applying audio processing by a DSP (Digital Signal Processor), etc. For example, Patent Document 1 describes a selector circuit that eliminates the need for analog switches, etc., that block output signals from playback units. In accordance with this selector circuit, a plurality of playback units receive a control signal and only a control unit that has been selected based upon the control signal is placed in the operating state; control units other than that placed in the operating state are made inoperative. Outputs from the playback units are supplied to and added by an adder circuit comprising an operational amplifier. In this case, only the playback units that have been placed in the operating state produce outputs; a playback unit that has been rendered inoperative does not produce an output. All outputs from playback units that have been placed in the operating state flow into the adder circuit. Accordingly, even if the output resistance of a playback unit rendered inoperative is low, the output from an operative playback unit will not be drawn into an inoperative playback unit. As a result, the output from a playback unit rendered operative will not be interfered with by a playback unit that has been rendered inoperative. This makes analog switches unnecessary.
The selector circuit described in Patent Document 1 operates so as to render an unused playback unit inoperative and add output signals from all playback units using an adder circuit. Consequently, it is required that the circuit operate in such a manner that each playback unit be rendered operative or inoperative. The result is a complicated circuit arrangement. Further, since an output signal from a playback unit that has been rendered inoperative is input to the adder circuit, there is the danger that noise from an inoperative playback unit will mix in.
Patent Document 2 discloses an integrating analog multiplexer that includes a plurality of multiplexer inputs, a multiplexer output, a switch device and a differential amplifier having an inverting input, a non-inverting input and an output. In this multiplexer the amplifier output forms the multiplexer output, the differential amplifier is connected as an inverting amplifier by a feedback path from the amplifier output to the inverting input, and the switch device selectively connects any one of the plurality of multiplexer inputs to the inverting input.
Patent Document 3 provides a space-saving and cost-effective on-chip solution for an audio output device and discloses a circuit that makes it possible to superimpose various audio input signals such as voice, music and a call signal. This circuit includes two or more audio input channels, a switch arrangement for driving different input audio signals, an arrangement of resistors for each of audio signal input lines, and an inverting voltage amplifier having inputs and an output and serving as an output stage for controlling superimposed audio signals. The input is coupled to a reference voltage and the audio signals are superimposed by an activated switch or switches. The output can be used in order to activate a loudspeaker. This circuit further includes input resistors for the voltage amplifier and means for setting the gain of the amplifier of the output stage.
As related art, Patent Document 4 discloses a gain control circuit capable of inputting a signal that has exceeded power-supply voltage and of changing the set range of gain. The gain control circuit is a circuit that operates as a simple power supply and has an amplifier so adapted that gain can be varied. The circuit has a plurality of input terminals to each of which is connected one end of a respective one of a plurality of resistance elements having weighted resistance values. The other ends of the plurality of resistance elements are tied together and the common connection node is connected to the input side of a variable resistance circuit for varying gain. It is possible to input a signal to all or some of the plurality of input terminals.    Patent Document 1: JP Patent Kokai Publication No. JP-A-8-79032    Patent Document 2: JP Patent Kokai Publication No. JP-P2003-198345A    Patent Document 3: JP Patent Kokai Publication No. JP-P2004-7590A    Patent Document 4: JP Patent Kokai Publication No. JP-P2003-229736A